Wool and wool-blend fabric treatment

ABSTRACT

The invention relates to a method of modifying the surface of a fabric which comprises the successive steps of: i) exposing the fabric surface to UV radiation; and ii) oxidative treatment of the fabric.

The present invention relates to wool and wool blend fabric treatmentsand in particular to novel methods of treating fabrics to give goodcolour yields when printed and/or to reduce pilling.

BACKGROUND

Wool and wool-blend fabrics have been processed and treated for manyyears to improve and/or enhance a wide range of characteristics. Forexample, the pre-treatment of fabrics, such as wool, before printing isessential to achieve good colour yields, levelness and brightness.Similarly a range of processes and treatments have been proposed toreduce or eliminate pilling.

Traditionally, chlorination has been used and several variants of thechlorination process are still used almost exclusively to prepare woolfabrics for printing. Dichloroisocyanuric acid (DCCA) is the most commonchlorination reagent currently in use, and can be applied by both batch(the most common) and continuous processes. The batch method involveschlorination with 3-4% DCCA on mass of fibre (omf), at pH 3.5-4.5 and atemperature of 20°-40° C. for about 1 hour, followed by an antichloraftertreatment with sodium bisulphite and acetic acid. The continuousprocess involves padding DCCA (35-50 gl⁻¹), followed by a dwell time of2-5 minutes before rinsing and an antichlor treatment similar to thebatch process. The alternative to DCCA is Kroy chlorination, originallyintroduced for treatment of wool tops, which uses a solution of chlorinegas in water in a continuous fabric treatment process. Chlorine reactswith water to give a mixture of hypochlorous and hydrochloric acids,which is sprayed directly onto the fabric with a wetting agent. Thereaction is more rapid than DCCA, but a rinsing and antichlor treatmentare still necessary. Processing speeds of 10-15 m min⁻¹ at a chlorinedose rate of 4% omf are typical and give similar performance to fabricstreated with 4% DCCA.

Typical problems with fabric chlorination include: yellowing, achievingan even application, and fibre damage. It is also very often necessaryto bleach chlorine-treated fabrics, usually with hydrogen peroxide, toremove yellowness before printing. However, it is the environmentalpressure on processes involving chlorine, particularly when absorbableorganohalogens (AOX) are present in the plant effluent, which is leadingto the replacement of chlorination by alternative technologies.

Other methods used to treat fabrics prior to printing are not common.Two polymer treatment routes, one for top and one for fabric arecurrently known, they are:

1. Hercosett 125 (trade name)

This polymer is applied to wool top after a prechlorination stage.Fabrics produced from treated top have an increased affinity for anionicdyes. The further mechanical processing which occurs during gilling,spinning and weaving results in a level preparation. However the colouryields tend to be lower since less chlorine is used. Further, care mustbe taken in washing off since treated wool has a high affinity for looseanionic dyes.

2. Synthappret BAP (trade name)

This polymer may be applied to a fabric without the need for aprechlorination step. The treatment of fabrics with this polymer priorto printing provides the fabric with a high affinity for hydrophobicdyes. However, the lack of a chlorination step reduces the penetrationof printing paste into the fibres, and control over the steamingconditions is critical. This method has been used to print wool/cottonblends, but not pure wool fabrics to date.

Other methods avoiding the use of chlorine have been developed but arenot considered to be commercially viable despite their reducedenvironmental impact. To summarise, the only prior art methods widelyused commercially for pretreating wool fabrics for printing involvechlorination, followed by rinsing and an antichlor treatment, which thenmay require a bleaching treatment to remove yellowness.

Pilling is a term used to describe the formation of small, tight ballsof fibre on a fabric surface. Pilling is highly detrimental to garments,resulting in a worn and unkempt appearance, and is a particular problemfor knitwear.

The pilling process is complex but can be described as four successivestages:

(i) Fuzz Formation. The mild rubbing action which occurs during wearteases some surface fibres from their parent yarns, resulting in a fuzzysurface.

(ii) Fuzz Entanglement. Areas of the garment which are subjected to morefrequent rubbing develop the higher fuzz densities. Fibres in such areasbecome entangled at some stage to form loose balls.

(iii) Pill Formation and Growth. Continued rubbing on looseentanglements causes some to roll into tighter balls. These tight ballsresist further rubbing forces, and some of the weaker fibres in thepills break. The stronger fibres remain intact and anchor the pills tothe fabric surface. Pills grow as they pick up loose fibres from thefabric surface.

(iv) Pill Wear-Off. The anchor fibres finally succumb to the steadilyincreasing forces acting on the pill and undergo fatigue failure. Aseach anchor fibre breaks, those remaining have to withstand largerforces and the rate of anchor failure thus accelerates. Pill removaloccures when the rate of anchor fibre breakage exceeds the rate of pillgrowth.

The nature of the fibres (origin, processing history, physicaldimensions), the yarn (type, twist) and the fabric structure are allimportant factors in pilling. In wear there are other variables whichcan influence the rate of pilling. It is well known that some wearersproduce more rapid and extensive pilling than others. Laundering cansubstantially alter pilling performance. Subjective differences betweenindividuals also exist over how objectionable a given amount of pillingis.

Several chemical treatments are known to reduce pilling, although as yetno process can guarantee zero pilling in wear. For example, theoxidative chlorination processes commonly used for shrinkproofing havesome beneficial effect. Chlorine/Hercosett and certain other polymertreatments which inhibit fibre migration by forming inter-fibre bonds,are also beneficial. More damaging dyeing conditions (i.e. long boilingtimes, high temperatures, extremes of pH) also tend to reduce pilling.

Similar to printing pretreatments there is currently a great deal ofenvironmental pressure against the use of processes which use chlorine,particularly when adsorbable organohalogens (AOX) are produced in planteffluent. Hence it is likely that the partially-effective anti-pillingtreatments and printing pre-treatments which involve chlorine compoundswill be phased out within the next ten years or so.

Applicant has now surprisingly found that the combination of subjectinga fabric to UV radiation followed by oxidative bleaching provides asynergistic mechanism to effectively increase the ability of the fabricto give good colour yield when printed and reduce the likelihood ofpilling.

Extensive investigations involving the use of either UV radiation oroxidative bleaching alone established that the single steps wereineffective in increasing colour yields or reducing pillingsignificantly. It was also established that the oxidative bleaching stepmust follow the irradiation, and cannot be applied first or duringirradiation while wet. It was found that high, even colour yields,better than those produced by 4% DCCA, were achieved using the two-stepprocedure over a range of classes of dye.

Most research on the effects of UV on wool has been aimed at limitingthe long-term negative effects such as photoyellowing, phototenderingand the fading of dyed wool. Previous work on the positive applicationof UV radiation (λ<400 nm) in the treatment of wool fabrics appears tobe limited to two commercial patents.

U.K. Patent 811702 describes the use of ultraviolet radiation formodifying the rate of dye uptake of wool fabrics. This increases thecolour yields of exposed fabric, depending on the nature of the dyeused. Use of suitable stencils during irradiation, followed by use of adye resist agent to partially protect unirradiated areas of fabricduring dyeing, can produce good tone-on-tone effects. This document alsodiscloses that in the interest of shortening the period of irradiationit is advantageous to treat the fabric with an oxidising agent during UVexposure. However, this document does not describe or suggest thepossible application of irradiation to fabric printing, or the method ofoxidative bleaching of the fabric after subjecting the fabric toirradiation. In fact, the U.K. patent stresses the use of an oxidisingagent during UV exposure to shorten the period of irradiation ratherthan as an essential, discrete step in a synergistic process to increasethe affinity of the fabric to dyes.

Japanese Patent H4-41768 claims that UV exposure alone is an effectiveshrinkproofing treatment for wool fabrics. However the claimed largereductions in fabric area shrinkage have not been reproduced in ourstudies. This could be due to the nature of the wool fabric used by theJapanese workers, or because their felting procedure was less severethan ours.

SUMMARY OF THE INVENTION

In particular, the present invention provides a method of modifying thesurface of a fabric which comprises the successive steps of:

(i) exposing the fabric surface to UV radiation; and

(ii) oxidative treatment of the fabric.

In the first step of the method of the invention the fabric may beirradiated by ultraviolet light from any suitable source. Preferably thefabric is subjected to ultraviolet radiation in the preferred range of400-180 nm. More preferably the fabric is subjected to short-wavelengthUV radiation (UV-C) having a wavelength of 280-200 nm and yet even morepreferably having a wavelength near the absorption maximum of thedisulphide bonds in wool (approximately 254 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the drawings accompanying the applicationwherein:

FIG. 1 is a graph comparing color yield with radiation time forUV-treated/peroxide bleached fabrics;

FIG. 2 is a graph comparing color yield with conveyor speed forUV-exposed/bleached fabric;

FIG. 3 is a report on pilling performance showing variation in the meannumber of pills per sample using ASTM random tumble test D3512-82; and

FIG. 4 is a pilling test according to ASTM random tumble test methodD3512-82 showing the variation of the mean number of pills observed forsamples in each group with tumbling time.

The UV radiation may be provided by any suitable source. The sourceselected will depend on the intensity and wavelength of irradiation tobe used in the method. Preferred sources of radiation for ultravioletradiation include low; medium-and high-pressure mercury arcs, and xenondischarge tubes. In a preferred embodiment of the invention alow-pressure mercury arc, producing 85% of emitted UV at 254 nm, may beused.

The length of time for which the fabric is irradiated will depend uponthe intensity and wavelength characteristics of the radiation source andthe desired result. Depending on the source of radiation, the length oftime required may range from a few seconds to 2 hours. For example, witha low intensity UV source, such as a low pressure mercury arc,irradiation times of 30-50 minutes may be required. With a suitablemedium or high-pressure mercury arc of high UV intensity (typically 120W cm⁻¹), irradiation times of a few seconds may be sufficient. Using asuitable elliptical or parabolic reflector to focus the UV radiationfrom a tube into a narrow strip or parallel beam allows fabric to betreated continuously, and this is clearly the most suitable commercialmethod for exposing large pieces of fabric. Alternatively a continuousirradiation process could be used to treat individual garments orlengths of fabrics for dyeing.

After UV exposure, the colour of the wool or wool-blend fabrics changefrom pale cream to pale olive-green, and this colour changes over anhour or so in room air to a pale yellow. Measurement by any conventionalmethod of the yellowness of fabrics irradiated with UV-C after standingfor 24 hours can be used to assess the degree of surface modification.

In the second step of the invention the fabric may be oxidised by anysuitable treatment. For example, it may be oxidised by using anysuitable oxidant such as hydrogen peroxide or permonsulphuric acid(PMS). In a preferred embodiment, the fabric is bleached using hydrogenperoxide. Preferably a solution of approximately 0.75% w/w hydrogenperoxide having a pH in the range 8-9 is used. The time period requiredfor bleaching will be dependent upon the type of fabric and oxidant usedand the desired result. The oxidant may be stabilised by any suitablestabiliser. For example, if a hydrogen peroxide solution is used thenthis may be stabilised by a tetrasodium pyrophosphate.

In a preferred embodiment of the invention only the UV exposure iscarried out continuously, followed by a batch bleaching treatment. TheUV-irradiated fabric can be stored for several months before bleachingwithout any reduction in colour yields or anti-pilling properties.

In another embodiment of the invention a fully continuous process usinga more rapid oxidant such as PMS may be used. It is also possible toundertake continuous bleaching by use of hydrogen peroxide pad/steammethods.

It is possible to create fine-detailed tone-in-tone effects on prints byplacing a suitable stencil between the radiation source and the fabricsurface After bleaching, the stencil design is invisible, but afterprint paste is applied irradiated areas take up more dye and show highercolour yields. Fine meshes, small repeating motifs or stripes can beused effectively to give the impression that a larger number of colourshave been used to produce the finished design.

It is also possible to transfer tone-in-tone designs from a compouter toa wool fabric. By using suitable graphic arts software, a complex designor caption can be cut into a thin adhesive PVC film which is opaque toUV radiation. The design is transferred either directly onto the woolfabric or onto a clear polyethylene or polypropylene film (which istransparent to UV down to 220 nm). After exposing the fabric to UV andbleaching, the design can be developed by overprinting a large area witha suitable dye paste.

EXAMPLES

The invention will now be described with reference to some specificexamples. Whilst the examples are limited to the treatment of woolfabrics prior to printing, this has been done for convenience and in noway is meant to limit the scope of the invention.

EXAMPLE 1

Pieces of scoured undyed shirting fabric were exposed to shortwavelength UV using a low pressure mercury arc (30 W) for periodsranging from 2-30 minutes by wrapping the fabrics around the UV tube.The fabrics were then bleached for one hour at 60° C. using 0.75% w/whydrogen peroxide solution stabilised by tetrasodium pyrophosphate (0.6%w/w) at pH 8-8.5. After rinsing, drying and steam pressing, the fabricwas printed using pastes of the following composition:

    ______________________________________                                        Indalca PA3, 10% stock solution                                                                     50%                                                     dye (e.g. Lanaset Blue 2R)                                                                           2%                                                     water                 38%                                                     urea                  10%                                                     ______________________________________                                    

Print pastes were prepared using Lanasol Black 5055, Lanasol Scarlet 3 Gand Drimarene Turquoise R-BLD dyes.

Test strips were printed using a Johannes Zimmer Sample Printing MachineType MDK, using two passes of a magnetic squeegee bar.

After printing, fabric was dried at room temperature, steamed at 100° C.for 30 minutes in an autoclave, washed off in warm water and dried. Allprints made on irradiated/bleached fabric were visibly more intense thanthose carried out using untreated, bleached only and UV-exposed onlyfabric. The reflectance spectra of printed samples was measured, and thereflectance values at the centre of the strongest absorption band wererecorded. These were converted to colour yield (K/S) values, which arerelated to the dye concentration at the surface, using the Kubelka-Munkequation.

The colour yield of UV-treated/peroxide-bleached fabric wassignificantly higher than that of unirradiated fabric. The colour yieldsincreased with irradiation time as shown in FIG. 1, and in all casesexceeded those for fabric treated with 4% dichloroisocyanuric acid(DCCA) after 30 minutes irradiation.

EXAMPLE 2

A piece fine glass fibre mesh was placed between a low pressure mercuryarc and a sample of ecru shirting fabric. The sample was exposed to UVfor 40 minutes, followed by peroxide bleaching as described inExample 1. The mesh design was not visible after bleaching, but afterprinting with Lanasol Black 5055, a fine-detailed black/greytone-in-tone effect was observed.

EXAMPLE 3

Scoured ecru wool fabric sampler were placed on a conveyor system andpassed below a medium pressure mercury arc the UV radiation from whichwas focused at the fabric surface using an elliptical reflector. Theconveyor speed was varied from 2 to 15 meters per minute, and a singleUV source having a power of 120 W/cm was used. Samples were given up tothree passes under the UV source over a range of conveyor speeds, tosimulate a machine having a series of UV tubes. The fabrics were thenbleached, printed with Lanasol Black 5055 and steamed as per Example 1,and the colour yields measured. The colour yields of UV-exposed/bleachedfabric varied with conveyor speed as shown in FIG. 2.

The example clearly demonstrates that colour yields better than a 4%DCCA treatment can be achieved using continuous UV irradiation operatingat speeds between 2 and 12 meters per minute.

EXAMPLE 4

A company logo was generated using computer graphic arts software andthe design was cut into a thin black adhesive PVC film. The design wasaffixed to a sheet of polyethylene film held taut on an aluminium frame.The frame was held firmly over a piece of wool challis fabric and a bankof low-pressure mercury arcs was positioned over the frame. The frameand fabric were exposed to UV for 40 minutes. The fabric was removed andbleached as described in Example 1. The entire area of the logo wasprinted with Drimarene Turquoise R-BLD paste, and the print was dried,steamed and washed off normally. Irradiated areas of the printed logowere far more intensely coloured than unexposed areas, and ahigh-quality tone-in-tone print was obtained.

EXAMPLE 5

Four groups of three standard pilling samples (double jersey knittedfabric) were prepared. The first group was exposed to UV-C radiationfrom a bank of eight low-pressure mercury tubes for 50 minutes on bothsides. The second group was also exposed to UV using similar conditions,but afterwards the samples were bleached for one hour at 60° C. withhydrogen peroxide (0.75% w/w) stabilised with tetrasodium pyrophosphate(6 g/l) at pH 8-8.5. The samples were rinsed well and allowed to dry.The third group of samples were bleached with peroxide only, and thefourth group of specimens were untreated controls. Pilling performancewas measured using an Atlas Random Tumble Pilling Tester (RTPT) usingthe standard procedure (ASTM D3512-82), with number of pills counted at5, 10, 15, 20, 25, 30 and 60 minute intervals. FIG. 3 shows thevariation in the mean number of pills per sample throughout the pillingtest. It is clear that only those samples treated with UV/peroxidebleaching show excellent anti-pilling performance.

EXAMPLE 6

Seven groups of three samples of standard double jersey knitted fabricwere prepared. The groups were exposed to UV-C radiation using anirradiator fitted with eight low-pressure germicidal UV tubes forperiods of 0, 5, 10, 20, 30, 40 and 50 minutes. All samples were thenbleached for one hour at 60° C. with hydrogen peroxide (0.75% w/w)stabilised with tetrasodium pyrophosphate (6 g/l) at pH 8-8.5. Thesamples were rinsed well in water and allowed to dry. The pilling testswere performed on each group of samples according to the standard ASTMrandom tumble test method (ASTM D3512-82), with number of pills countedat 5, 10, 15, 20, 25, 30 and 60 minute intervals. FIG. 4 shows thevariation of the mean number of pills observed for samples in each groupwith tumbling time. Clearly the extent of UV irradiation has a dramaticeffect on the degree of pilling observed; zero pilling was foundthroughout the pilling test for all samples irradiated with UV for 50minutes.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention withoutdeparting from the spirit and scope of the invention as broadlydescribed. The present embodiments are, therefore, to be considered inall respects as illustrative and not restrictive.

I claim:
 1. A method of modifying the surface of a wool or wool blendfabric which comprises the successive steps of:i) exposing the fabricsurface to UV radiation; and ii) oxidatively treating the fabric byexposing the fabric to an aqueous oxidative solution.
 2. A methodaccording to claim 1 wherein the fabric is exposed to UV radiation inthe range of 400-180 nm.
 3. A method according to claim 2 wherein thefabric is exposed to UV radiation in the range of 280-200 nm.
 4. Amethod according to claim 1 wherein the fabric is exposed to UVradiation for a time period in the range of a few seconds to 2 hours. 5.A method according to claim 1 wherein in step i) the fabric is exposedto UV radiation through a stencil.
 6. A method according to claim 5wherein the stencil is generated using computer graphic arts software toproduce a stencil having UV-transparent and UV-opaque components.
 7. Amethod according to claim 1 wherein the oxidative solution compriseshydrogen peroxide or permonosulfuric acid.
 8. A method according toclaim 5 wherein the fabric is bleached by an oxidative solution of 0.75%w/w hydrogen peroxide and having a pH in the range 8-9.
 9. A methodaccording to claim 1 wherein the oxidative solution is stabilized bytetrasodium pyrophosphate.
 10. A method according to claim 1 whereinstep i) is carried out continuously.
 11. A method according to claim 1wherein step ii) is carried out in batch mode.
 12. The method of claim1, wherein after said step i) the fabric has a pale yellow color. 13.The method of claim 1, wherein said method reduces fabric pilling. 14.The method of claim 1, wherein said fabric surface is exposed to UVradiation for about 40 to 60 min.